Switching power supplies, also known as power converters or switching converters, often use one or more magnetic elements, such as transformers or inductors, to process power or energy from an input source and deliver it to an output load. In many applications, isolated converters, which provide electrical isolation between the input source and output load, are desirable or required by various regulations. In isolated converters, a transformer may be used as the isolation element (i.e., an isolation transformer). In such applications the transformer provides electrical isolation between the input/primary side of the transformer and the output/secondary side of the transformer. In many applications, the isolation transformer may also serve as the magnetic element of the converter. Examples of such converters include flyback converters and isolated forward converters.
In any power converter, power is transferred forward from source to load. Effective control of the converter may also require a feedback loop from load to source so that appropriate output electrical characteristics, e.g., voltage and/or current, can be maintained. Thus, a feedback loop may be established between the load and input to control and adjust the power flow as required. This results in what is known as closed loop operation, and many control techniques for closed loop operation are known in the art. In general, these closed loop control techniques include a reference device that sets a desired value for a regulated output parameter (such as an output voltage setpoint), a sensor that senses the value of the output parameter to be regulated, and a mechanism for comparing reference to the sensed value and altering operation of the converter to make the sensed value match the reference. Exemplary closed loop controllers include, proportional controllers, proportional-integral (PI) controllers, proportional-integral-derivative (PID) controllers, hysteretic controllers, digital controllers, etc.
For non-isolated switching converters closing the feedback control loop is straight-forward. A direct electrical connection can suffice because the input and output are referred to the same electrical ground, and the switching controller (PWM controller) and the feedback reference and network are all in the same place. However, for isolated converters, feedback control requires a mechanism for safely getting the sensed output signal across the isolation barrier provided by the transformer. Two common solutions are magnetic coupling (using an additional signal transformer or one or more auxiliary windings of the power transformer) and optical coupling. Both techniques have drawbacks. With respect to magnetic coupling, separate signal transformers are bulky, expensive, and complicate controller design because of the impacts their performance has on the feedback signal. Auxiliary windings may complicate as well as compromise design of the power transformer while also causing controller complication issues similar to a separate signal transformer. Although opto-couplers are usually cheaper and smaller, they are bandwidth limited, subject to high levels of variation with response to temperature, and subject to wide variations in gain and other performance metrics from piece-to-piece, lot-to-lot, and vendor-to-vendor.
Because of these and other limitations in providing feedback loops in isolated power converters, isolated converters to date have largely used relatively straightforward PID loop control, which has limited the ability of power converter designers to adopt higher performing control techniques that could potentially maximize power converter performance. Thus, what is needed in the art are improved feedback techniques for isolated power converters.